Sulfur-containing compounds are present to a greater or lesser extent in petroleum and in the refined products from petroleum. Organic sulfur compounds are undesirable in petroleum products for several reasons. They can poison catalysts used in petroleum refining. In addition, the greater the amount of organic sulfur compounds in hydrocarbon fuels, such as gasoline and heating oil that are products of refining, the greater the potential for pollution when these fuels are burned. Such pollution can be reduced by treating the emissions, but treatment increases costs, and organic sulfur compounds in automotive exhaust are known poisons for automotive catalytic converters, reducing both their efficiencies and their lifetimes. Though organic sulfur compounds have always been present in petroleum and its products, the problems posed by organic sulfur compounds to refiners and consumers have increased with time. First, the demand for petroleum products is leading to the exploitation of sources of petroleum that had been previously passed over as undesirable because of high organic sulfur content. Second, more stringent environmental regulations are reducing permissible levels of sulfur in emissions and in fuels.
Methods have been developed over the years to deal with the problems posed by organic sulfur compounds in petroleum. These range from simple treatments aimed at rendering less offensive the malodorous sulfur compounds, to more complex processes to reduce or remove the sulfur compounds. The latter include absorption, extraction, oxidation, and hydrodesulfurization. A description of these processes in detail can be found in U.S. Pat. No. 4,383,916. Hydrodesulfurization is the most widely used of these processes and is growing in importance as the demand for fuels with low sulfur content increases. However, hydrodesulfurization is energy intensive, consumes expensive hydrogen gas, and requires high pressure vessels and piping. Furthermore, non-sulfur compounds in the hydrocarbon stream can be changed by hydrogenation or hydrogenolysis, and this is not always desirable. For example, under certain conditions aromatic and olefinic compounds may be hydrogenated.
In an attempt to reduce the organic sulfur concentration in hydrocarbon streams without the undesirable costs and side effects of hydrodesulfurization, acid catalysts have been employed. Acids can react with organic sulfur compounds to convert the sulfur to forms, such as hydrogen sulfide, which can be cheaply and easily removed from petroleum streams. A number of patents have disclosed acidic zeolites as catalysts for desulfurization without the accompanying use of hydrogen gas, for example, U.S. Pat. Nos. 4,383,916, 5,401,391, and 5,482,617. Zeolites are limited, however, because their acid strength is not sufficiently high for effective desulfurization. Furthermore, it can be difficult to remove fouling from the zeolites and their sensitivity to alkali limits the range of cleaning agents that may be used for this purpose.